1. Field of the Invention
This invention relates to flat panel devices such as a flat cathode ray tube. More particularly, this invention relates to a support structure for supporting a faceplate and backplate of a flat panel device against the force arising from the differential pressure between a vacuum pressure within the flat panel device and the external atmospheric pressure.
2. Related Art
Numerous attempts have been made in recent years to construct a flat cathode ray tube (CRT) display (also known as a "flat panel display") to replace the conventional CRT display in order to provide a lighter and less bulky display. In addition to flat CRT displays, other flat panel displays, such as plasma displays, have also been developed.
In flat panel displays, a faceplate, a backplate, and connecting walls around the periphery of the faceplate and backplate form an enclosure. In some flat panel displays, the enclosure is held at vacuum pressure, e.g., in flat CRT displays, approximately 1.times.10.sup.-7 torr. The interior surface of the faceplate is coated with phosphor or phosphor patterns which defines the active region of the display. Cathodes located adjacent the backplate are excited to release electrons which are accelerated toward the phosphor on the faceplate. When the phosphor is struck by electrons, the phosphor emits light which is seen by a viewer at the exterior surface of the faceplate (the "viewing surface").
A force is exerted on the walls of the flat panel display due to the differential pressure between the internal vacuum pressure and the external atmospheric pressure that, left unopposed, can make the flat panel display collapse. In rectangular displays having greater than an approximately 1 inch diagonal (the diagonal is the distance between opposite corners of the active region), the faceplate and backplate are particularly susceptible to this type of mechanical failure due to their high aspect ratio (here, either the width or the height divided by the thickness).
One way to increase the resistance of the faceplate and/or backplate to collapse is to form the faceplate and/or backplate in an arced shape to increase the ability of the faceplate and/or backplate to carry the applied load. However, such arcing makes the overall display undesirably thick.
Another way to increase the resistance of the faceplate and/or backplate to collapse is to make the faceplate and/or backplate relatively thick. However, this is undesirable because of the added weight and bulk attendant the increased thickness, and because of contrast and resolution problems, such as increased spot-halo and light-spreading problems, due to internal reflections within the thick faceplate.
Since the faceplate and backplate comprise a significant fraction of the total volume of material required to make a flat panel display, it is desirable to use thin, lightweight material for both the faceplate and backplate. Thus, there is a need for a flat panel display having a means of supporting a thin, lightweight faceplate and backplate against the pressure differential existing across the faceplate and backplate.
Spacers have been used to support the faceplate and/or backplate. Previous spacers have been walls or posts located between pixels (phosphor regions that define the smallest individual picture element) in the active region of the display. However, the presence of the spacers may adversely affect the flow of electrons toward the faceplate. Additionally, the spacers must be constructed so that they are not visible on the external viewing surface of the display.
Previously, spacers have been formed by photopatterning polyimide. However, for large aspect ratios (here, the length of the spacer, in a direction perpendicular to the faceplate and backplate, divided by the thickness of the spacer), e.g., greater than 4:1, photopatterned polyimide spacers are not sufficiently strong to withstand the loads applied and are susceptible to buckling or deforming. Since the aspect ratio of photopatterned polyimide spacers must be relatively small, in flat panel displays using such spacers, the spacing between backplate and faceplate is reduced, requiring that low voltage (i.e., less efficient) phosphor be used on the faceplate.
Additionally, polyimide has a coefficient of thermal expansion that cannot be adequately matched to the coefficient of thermal expansion of the materials typically used for the faceplate, backplate and addressing grid, i.e., glass for the faceplate, glass, ceramic, glass-ceramic or metal for the backplate, and glass-ceramic or ceramic for the addressing grid. Therefore, heating that occurs during assembly of the flat panel display, as well as heating that may occur during use of the flat panel display, can cause a different amount of expansion and contraction of the spacers, relative to the addressing grid, faceplate and/or backplate, that results in registration problems between the spacers and the addressing grid, faceplate and/or backplate, or damage to the faceplate or backplate.
Finally, when used in a vacuum pressure environment, such as is present within a flat panel display, polyimide spacers may be susceptible to outgassing as a result of electrons colliding with the spacers.
Spacers have also been made of glass. However, glass may not be as strong as desired. Further, micro-cracks that are inherent in glass make glass spacers even weaker than "ideal" glass because of the tendency of micro-cracks to propagate easily throughout glass.